To test a given fiber optic link, an OTDR is connected to one end of this link and then set up to inject a series of light pulses into the link while sampling (measuring) returned light power as a function of time. Returned light is composed of (Rayleigh) backscatter from the optical fiber sections comprising the link and (Fresnel) reflections from connections, splitters, breaks, or other events within the link. By converting sample times into distances based the estimated speed of light in optical fiber, the OTDR can create a graphic representation of link insertion loss as a function of distance and the reflectance of link events.
OTDR performance is generally specified in terms of measurement accuracy, measurement range, ability to resolve and measure closely-spaced events, measurement speed, and ability to perform satisfactorily tinder various environmental extremes and after various types of physical abuse. In addition, OTDR value is impacted by its price and user experience factors such as size, weight, and ease of use.
Measurement accuracy is defined as the uncertainty of a measurement, or the difference between a measured value and the true value of the parameter being measured. OTDR measurement range is defined as the maximum link insertion loss at which a specified parameter of a specified event type can be measured with a specified accuracy. Resolution is a measure of how close two events can be spaced and still be accurately measured, or at least recognized, as separate events. OTDR resolution is a function injected light pulse duration and returned light measurement bandwidth. In general, shorter injected light pulse duration and higher returned light measurement bandwidth will result in better (i.e. shorter) OTDR resolution. Resolution is also impacted by event loss and reflectance. In general, the larger the loss or reflectance of the first event in an event pair, the longer the distance between the events must be to ensure that the second event can be measured with a specified accuracy.
An event pair directly related to the present invention are the two connections created when an OTDR is connected to the link under test at a patch panel using a fiber optic test cord. The first connection in this pair occurs at the OTDR test port. The second occurs at the first connector of the link under test. If the OTDR test port connection had zero loss and zero reflectance, then any OTDR could accurately measure the first link connection regardless of test cord length. However, the best quality, lowest-reflectance fiber optic connectors available today have some loss and reflectance. As a result, even OTDRs with state of the art resolution performance require a test cord length of about 50 meters or more to ensure accurate measurement of the first link connection.
Because such long jumpers would be hard to use and expensive, OTDR manufacturers and other vendors offer special test cord assemblies called Launch Cables, which comprise a coil of optical fiber, enclosed in a protective housing, with connectorized flying leads—in effect one half of a fiber optic jumper—at each end, and a total connection-free fiber length sufficient to ensure accurate measurement of the first link connection.
One such assembly is described in U.S. Pat. No. 6,915,058 document that is published at Jul. 5, 2005. In this document is described a retractable Optical Fiber assembly which includes a housing, a spring-loaded, rotatable spool, and a length of optical fiber reeled onto the spool. The optical fiber comprises a central length of thin (unjacketed) fiber terminated by 1 or 2 m of jacketed fiber at each end. This product is designed so that the jacketed fiber end sections, essentially two fiber optic jumpers fusion spliced to the central fiber section, are reeled on to and off of the spool in the same direction. The total length of optical fiber is greater than the minimum optical fiber length needed to ensure accurate measurement of the first link connection.
One problem of this solution is that it requires the OTDR user to carry a separate box with their OTDR. In addition, each time the OTDR is unpacked for use, this launch cable embodiment must be connected to the OTDR test port thus exposing the OTDR test port and a launch cable connector to unnecessary wear and possible damage if the two connectors are not cleaned properly before they are connected.
To address the two problems described above, the OTDR can be equipped with an internal launch cable. For example, in the embodiment described in U.S. patent US20050259242, published on Mar. 21, 2006, the OTDR end of a “smart” fiber test module (SFTM) is fusion-spliced to the OTDR while other end is equipped with a flying lead, basically a fiber jumper, so that the SFTM can be connected to a patch panel. The OTDR and SFTM share a common housing and thus may be considered a single instrument or unit.
However this solution presents a new and significant problem. Because the SFTM is fusion spliced to the OTDR there is no easy way for users to replace this module in the event that its flying lead (jumper), or the connector on this lead, becomes damaged through misuse, or simply worn out over time from normal use.
A similar solution can be found in OTDRs with built-in launch cables, or more correctly launch fibers, such as the Anritsu MT9090A OTDR: http://www.anritsu.com/en-US/-products-Solutions/Products/MT9090A.aspx. However, this embodiment suffers the same problem as the one noted above. Because it is fusion-spliced to the OTDR it cannot be replaced easily. In addition, because no flying lead is provided, this solution can only be used to test links with end cables that are not terminated in a patch panel.
In contrast, the present invention as described in the next sections, solves the problems described above related to separate launch cable assemblies without adding problems associated launch cables that are fusion spliced to the OTDR, or that have no flying lead. In addition the present invention offers several new advantages.